Tensile testing of materials is nowadays one of the most common ways to measure and check material dependent properties. Such tests can give almost any material data a designer need to know.
For tensile testing a tensile testing specimen is normally mounted between two attachment devices. One of the devices is normally arranged at a frame, while the other is arranged at a movable pulling yoke. The pulling yoke is displaced so as to elongate the tensile testing specimen, which finally breaks. The most interesting quantities to be measured is the strain of the tensile testing specimen and the tensile force of the pulling yoke. The relation between strain and tensile force may be recorded as the pulling continues. Other interesting quantities, often required to be determined is the fracture area, the maximum tensile force, and the strain at fracture.
In tensile testing devices according to the state of the art the tensile testing specimens are attached by wedging jaw means. These operate so as to fasten the tensile testing specimen harder as the tensile force increases. Such arrangements often give a simple and safe attachment without need for external fastening mechanisms, but have the inherent disadvantage that the tensile testing specimen moves slightly with respect to the fastening devices during the tensile testing itself, and in particular at its beginning. This disadvantage results in that the displacement of the fastening devices is unable to give any true measurement of the strain of the tensile testing specimen, why other means generally are used for strain measuring.
Strain sensors, which are in contact with the tensile testing specimen, are a common type of measuring devices. They may consist of pure mechanical arrangements, electromagnetic devices or strain gauges. Common to all these types of strain sensors is that they have to be attached with accuracy at the tensile testing specimen, which makes them inconvenient for automatic testing constructions. Furthermore, there is a large risk that the sensors are broken or damaged when the tensile testing specimen reaches fracture.
Contact-free strain sensors are more suited for automatic testing, since the above mentioned disadvantages are avoided. The most common methods are to, on the tensile testing specimen, scribe or in any other manner apply a mark, whose displacement then may be followed by different recording instruments. In the patent publication U.S. Pat. No. 5,193,398, a device for strain measuring is disclosed, which uses a recording of a lattice by means of a CCD camera. Common for the contact-free strain sensors according to the state of the art is that they all require some kind of marking of the tensile testing specimen, which on one hand involves an extra operation and on the other hand may run the risk of changing the properties of the tensile testing specimen somewhat.
Tensile testing according to the state of the art operates almost exclusively with a constant pulling speed, which is achieved by adjusting the tensile force. However, in a tensile test curve, the most important information is to find in the areas where the change of the force is large, which results in that the accuracy of measurements of certain measures will suffer, since these regions are passed rapidly. On the contrary, there are many parts in a tensile test curve, which are fairly uninteresting, but where the force change is small, why these areas are passed slowly. Accordingly, the time of the tensile testing is used very inefficiently.
One area of particular interest is the area immediately before fracture. For tensile testing according to the state of the art, this part is passed rapidly and only a little information is available by the tensile test curve. One interesting measure is the area of the fracture, and according to the state of the art, this area is measured by hand after the completion of the fracture. It is obvious that a manual step of operation of this kind introduces large uncertainties in the measuring, particularly as the surface of the fracture is irregular. The procedure is also comparatively time consuming. For many tensile testing specimens, the surface of fracture is not simply definable, why different models for making the measurement will influence the result.
The quantities which are measured during tensile testing are according to the state of the art related to the original area of the tensile testing specimen. The measure of the area varies during the tensile testing and knowledge about this variation is interesting in many aspects. No tensile testing equipment according to the state of the art can provide such an information.
By tradition, the majority of the tensile testing equipments are arranged in a vertical configuration, often leading to unergonomic working positions at sample mounting, adjustment and measuring. Furthermore, a vertical configuration is difficult to combine with an automatic feeding of tensile testing specimens.